doc: Minor readme structure improvements

Improves wording and sentence structure of the readme, including:

- Using "TF-A" in place of "the TF-A" in a couple of places
- "Support for X" in place of "Support X"
- Explicitly naming the Secure Payload Dispatcher when introducing the
  SPD acronym.

Change-Id: I4e7b7e7dc6748f631d07c02d97adcca10ae60ea4
Signed-off-by: Paul Beesley <paul.beesley@arm.com>